Defender Paul Downing cannot afford to keep making mistakes at the back, according to MK Dons manager Karl Robinson, after his error gifted Bury the opening goal in Tuesday night's 3-1 defeat.
The 24-year-old moved to Stadium MK from Walsall in the summer, forming a new defensive partnership with Joe Walsh following the departures of Kyle McFadzean and Antony Kay.
But he has found life tough at the back as Dons have made a torrid start to life in League 1, and another mistake on Tuesday night handed James Vaughan his first of the night after just eight minutes.
While Dons fought vehemently to get an equaliser just after the hour mark, Vaughan struck again before Zeli Ismail's runaway goal in stoppage time completed the 3-1 win for the Shakers.
But Downing's mistake so early in the game left Dons on the back foot, and his manager says the defender must do better of he wants him to keep faith.
"He came in in the summer to strengthen us, but he needs to be better than that," said Robinson. "It happened against Oxford and he got away with it, and he nearly did it at Fleetwood with Devante Cole.
"He has to learn from it if he wants to be a better player. He just had to do something with it. I can't think for him - all I can do is set us up to play against a team second in the division.
"Paul held his hands up for the opener, but what can we do about that? He owned up to his error. That's just an individual component."
While Dons had 17 shots on the night, hit both posts and forced incredible saves from Bury keeper Ben Williams, the visitors needed just a handful of efforts to put three past Dons, who are now without a win in 10 games at Stadium MK.
Robinson said: "It was a smash and grab game. We can't believe the result. I can't ask for more effort, or for us to create more chances.
"What can we do when it hits the post and goes through Nicky Maynard's legs? We were denied a penalty after a foul on Kabongo, the keeper has made one of the best save of his life too.
"It's just poor decision making when things are a little bit edgy. As a manager, what more can I do?"